A wireless base station and a host device, which aggregates wireless base stations, are connected via an optical section (also referred to as an “optical transmission section”) including an optical device and an optical fiber. This optical section is referred to as mobile backhaul. Also, systems for separating an antenna unit (a remote radio head (RRH)) of a wireless base station and a signal processing unit (a baseband unit (BBU)) have recently been widely considered. The RRH and the BBU are connected via an optical device and an optical fiber, and an optical section thereof is referred to as mobile fronthaul.
FIG. 8 is a diagram illustrating a configuration example of mobile fronthaul and mobile backhaul. FIG. 8 illustrates a wireless communication system in which a large number of small cells with low transmission, power are overlaid in the coverage of a macro cell with high transmission power as an example. An optical wavelength division multiplexing (WDM) mode is used for the mobile backhaul between a wireless base station and a host device and the mobile fronthaul between an RRH and a BBU. In the WDM mode, different wavelengths are used for uplink and downlink. It is possible to transmit and receive an uplink signal and a downlink signal at the same time by a single core optical fiber using the WDM mode (for example, see Non-Patent Literature 1). Also, the uplink is a link in a direction from a terminal (a mobile wireless terminal in the case of the wireless communication system) to a network (a mobile network (NW) in the case of the wireless communication system) and the downlink is a link in a direction from the network to the terminal.
On the other hand, in wireless communication system, two modes of a frequency division duplex (FDD) mode and a time division duplex (TDD) mode are widely used. In the FDD mode, different frequency bands are used for uplink and downlink. In the TDD mode, the uplink and the downlink share the same frequency band and signals are orthogonal on a time axis.
FIG. 9 is a diagram illustrating a TDD frame of Long Term Evolution (LTE). As illustrated in FIG. 9, for example, seven types of TDD frames are set in LTE, and each TDD frame includes nine subframes. In the TDD mode, a frame configuration is switched in accordance with uplink and downlink traffic. Through this switching, a ratio of communication times of the uplink and the downlink can be flexibly set. A special subframe (S) illustrated in FIG. 9 includes a downlink pilot time slot (DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS) (for example, see Non-Patent Literature 2). DwPTS is a time slot used for transmission of a downlink control signal. UpPTS is a time slot used for transmission of an uplink control signal. GP is a protection time used for conversion from downlink into uplink.
FIG. 10 is a diagram illustrating an example of bandwidth utilization in a wireless section and an optical section when a wireless communication system uses the TDD mode. In a wireless section in which a wireless base station or an RRH of the wireless communication system communicates with a mobile wireless terminal, data is transmitted so that uplink and downlink are orthogonal on a time axis with the same frequency f1. Thus, even when data to be transmitted and received in the wireless section is relayed in the optical section through downlink of a wavelength λ1 and uplink of a wavelength λ2, the uplink and the downlink are transmitted to be orthogonal on a time axis. Accordingly, there is a section in which data transmission is hardly performed at the wavelengths of the uplink and the downlink even though the WDM mode in which the uplink and the downlink can communicate simultaneously is adopted in the optical section.
Also, it is conceivable that a passive optical network (PON) system, which is one of optical transmission systems, accommodates a communication system such as a wireless communication system.
FIG. 11 is a diagram illustrating an example of a communication system using a PON system in an optical section. Conventionally, the PON system generally accommodates a service such as a fiber to the home (FTTH) service. Recently, accommodation of a machine to machine (M2M) service in which devices communicate with each other and operate without human intervention has also been considered. However, as illustrated in FIG. 11, it is conceivable that a PON system accommodating the FTTH service and the M2M service and a PON system accommodating the wireless communication system are physically separate networks.